In association with recent increasing tendency toward higher operation speed and higher functionality of semiconductor integrated circuit devices (IC chips) used as, for example, microprocessors of computers, the number of terminals increases, and the pitch between the terminals tends to become narrower. Generally, a large number of terminals are densely arrayed on the bottom surface of an IC chip and flip-chip-bonded to terminals provided on a motherboard. However, since the terminals of the IC chip differ greatly in pitch from those of the motherboard, difficulty is encountered in bonding the IC chip directly onto the motherboard. Thus, according to an ordinarily employed method, a semiconductor package configured such that the IC chip is mounted on an IC chip mounting wiring substrate is fabricated, and the semiconductor package is mounted on the motherboard.
The IC chip mounting wiring substrate which partially constitutes such a semiconductor package is practicalized in the form of a multilayer substrate configured such that a build-up layer is formed on the front and back surfaces of a substrate core. The substrate core used in the multilayer wiring substrate is, for example, a resin substrate (glass epoxy substrate) formed by impregnating reinforcement fiber with resin. Through utilization of rigidity of the substrate core, resin insulation layers and conductive layers are laminated alternately on the front and back surfaces of the substrate core, thereby forming respective build-up layers. In the multilayer wiring substrate, the substrate core serves as a reinforcement and is formed very thick as compared with the build-up layers. Also, the substrate core has conductor lines (specifically, through-hole conductors, etc.) extending therethrough for electrical communication between the build-up layers formed on the front and back surfaces.
In recent years, in association with implementation of high operation speeds of semiconductor integrated circuit devices, signal frequencies to be used have become those of a high frequency band. In this case, the conductor lines which extend through the substrate core serve as sources of high inductance, leading to the transmission loss of high-frequency signals and the occurrence of circuitry malfunction and thus hindering implementation of high operation speed. In order to solve this problem, a multilayer wiring substrate having no substrate core is proposed (refer to, for example, Patent Documents 1 and 2). The multilayer wiring substrates described in Patent Documents 1 and 2 do not use a substrate core, which is relatively thick, thereby reducing the overall wiring length. Thus, the transmission loss of high-frequency signals is lowered, whereby a semiconductor integrated circuit device can be operated at high speed.
In the manufacturing method disclosed in Patent Document 1, a metal foil is disposed one side of a provisional substrate, and a plurality of conductive layers and a plurality of resin insulation layers are alternately stacked on the metal foil to thereby form a build-up layer. Subsequently, the metal foil is separated from the provisional substrate so as to obtain a structure in which the build-up layer is formed on the metal foil. The surface of the outermost layer (the surface of a resin insulation layer and the surfaces of a plurality of IC-chip connection terminals) is exposed by means of removing the metal foil through etching, whereby a multilayer wiring substrate is manufactured.
Patent Document 1 also discloses a multilayer wiring substrate in which a solder resist film is formed on the build-up layer as the outermost layer thereof. Notably, openings are formed in the solder resist film so as to expose the surfaces of IC-chip connection terminals. In a multilayer wiring substrate disclosed in Patent Document 2 as well, a solder resist film is formed, as the outer most layer, on the side of the wiring substrate where an IC chip is mounted, and openings are formed in the solder resist film so as to expose the top surfaces of IC-chip connection terminals. The solder resist film is made primarily of a hardened photocurable resin insulation material. The openings of the solder resist film are formed through exposure and development performed in a state in which a predetermined mask is disposed on the solder resist film. Subsequently, solder bumps are formed on the top surfaces of the IC-chip connection terminals exposed within the openings of the solder resist film, and an IC chip is mounted via the solder bumps.
Furthermore, there has been proposed a multilayer wiring substrate for mounting an IC chip in which the space between an IC chip connected to the IC-chip connection terminals and the surface of the substrate is sealed by use of an underfill agent.    Patent Document 1: Japanese Patent Application Laid-open (kokai) No. 2007-158174    Patent Document 2: Japanese Patent Application Laid-open (kokai) No. 2004-111544